runtime: disable CPU profiling before removing the SIGPROF handler

Otherwise, in c-archive or c-shared mode, there is the chance of
getting a SIGPROF just after the signal handler is removed but before
profiling is disabled, in which case the program will die.

Fixes #46498

Change-Id: I5492beef45fec9fb9a7f58724356d6aedaf799ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/329290


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>